Cross-reference is hereby made to commonly assigned related U.S. Applications, filed concurrently herewith, Ser. No. 10/124,802, entitled xe2x80x9cINSULATING MEMBER FOR A MEDICAL ELECTRICAL LEAD AND METHOD FOR ASSEMBLYxe2x80x9d; 10/124,185, entitled xe2x80x9cDRIVE SHAFT SEAL FOR A MEDICAL ELECTRICAL LEADxe2x80x9d; 10/124,530, entitled xe2x80x9cIMPLANTABLE MEDICAL LEAD HAVING A RETRACTION STOP MECHANISMxe2x80x9d; and 10/124,160, entitled xe2x80x9cAPPARATUS FOR TRANSFERRING TRACTION FORCES EXERTED ON AN IMPLANTABLE MEDICAL LEADxe2x80x9d.
The present invention relates to a medical electrical lead, and, more specifically, relates to an implantable medical lead system that is readily manufactured with improved reliability.
A wide assortment of automatic, implantable medical devices (IMDs) are presently known and in commercial use. Such devices include cardiac pacemakers, cardiac defibrillators, cardioverters, neurostimulators, and other devices for delivering electrical signals to a portion of the body and/or receiving signals from the body. Pacemakers, for example, are designed to operate so as to deliver appropriately timed electrical stimulation signals when needed, in order to cause the myocardium to contract or beat, and to sense naturally occurring conduction signals in the patient""s heart.
Devices such as pacemakers, whether implantable or temporary external type devices, are part of a system for interacting with the patient. In addition to the pacemaker device, which typically has some form of pulse generator, a pacing system comprises one or more leads for delivering generated stimulation pulses to the heart and for sensing cardiac signals and delivering sensed signals from the heart back to the pacemaker. As is known, pacemakers can operate in either a unipolar or bipolar mode, and can pace the atria or the ventricles. Unipolar pacing requires a lead having only one distal electrode for positioning in the heart, and utilizes the case, or housing of the implanted device as the other electrode for the pacing and sensing operations. For bipolar pacing and sensing, the lead typically has two electrodes, a tip electrode disposed at the distal end of the lead, and a ring electrode spaced somewhat back from the distal end. Each electrode is electrically coupled to a conductive cable or coil, which carries the stimulating current or sensed cardiac signals between the electrodes and the implanted device via a connector.
Combination devices are available for treating cardiac arrhythmias that are capable of delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or xe2x80x9cICDxe2x80x9d, uses coil electrodes for delivering high-voltage shock therapies. An implantable cardiac lead used in combination with an ICD may be a quadrapolar lead equipped with a tip electrode, a ring electrode, and two coil electrodes. A quadrapolar lead normally requires four conductors extending the length of the lead body in order to provide electrical connection to each electrode, potentially resulting in a substantial increase in lead body diameter.
Other leads used with ICDs may be tripolar or bipolar. A tripolar lead that is also known as a xe2x80x9cdedicated bipolarxe2x80x9d lead is configured with a tip electrode, a ring electrode and a coil electrode. The tip and ring electrodes serve as a bipolar sensing pair. The coil electrode serves as the defibrillation electrode, and the tip electrode serves as the pacing electrode. An xe2x80x9cintegrated bipolarxe2x80x9d lead, also used with ICDs, is configured with a tip electrode and a coil electrode but no ring electrode. The tip and coil electrodes serve as a bipolar pair for sensing and each serve individually as unipolar pacing and defibrillation electrodes, respectively. Each of these types of leads has different advantages related to the size of the lead, the location of the electrodes after implantation, and the characteristics of the sensed cardiac signals.
In order to work reliably, cardiac leads need to be located at a targeted cardiac tissue site in a stable manner. One common mechanism for securing an electrode position is the use of a rotatable fixation helix. The helix exits the distal end of the lead and can be screwed into the body tissue. The helix itself may serve as an electrode or it may serve exclusively as an anchoring mechanism to locate an electrode mounted on the lead adjacent to a targeted tissue site. The fixation helix may be coupled to a drive shaft that is further connected to a coiled conductor that extends through the lead body as generally described in U.S. Pat. No. 4,106,512 to Bisping et al. A physician may rotate the coiled conductor at its proximal end to cause rotation of the fixation helix via the drive shaft. As the helix is rotated in one direction, it is secured in the cardiac tissue. Rotation in the opposite direction removes the helix from the tissue to allow for repositioning of the lead at another location.
One problem that can arise with the use of a fixation helix is over-retraction of the helix during lead repositioning. Repositioning of the lead may be required during an implant procedure if poor electrical contact is made with the targeted cardiac tissue, resulting in higher than desired stimulation thresholds or poor sensing. The physician must retract the helix by applying turns to the coiled conductor in the appropriate direction. The physician may not have tactile feedback or fluoroscopic image indicating when the helix has dislodged from the heart tissue and is fully retracted. In many cases, the physician will perform additional turns of the coiled conductor in order to ensure the helix is safely removed from the heart tissue before applying tension to the lead to relocate it. Excessive turns, however, can cause deformation of the fixation helix rendering it unusable. In such cases, the lead must then be removed and replaced by a new lead.
To address the problem of over-retraction, a retraction stop mechanism may be provided within the distal lead head. An exemplary retraction stop mechanism that includes a fixed stop formed of a plurality of fixed cam and axial stop surfaces and a movable stop formed of a like plurality of rotatable cam and axial stop surfaces is disclosed in U.S. Pat. No. 5,837,006 to Ocel et al.
When using a lead having an open tip to allow for advancement and retraction of a fixation helix, it is desirable to prevent the ingress of body fluids into the lead body. Blood or other body fluids entering the lead body can create a pathway for infection, a serious complication with implantable devices. Furthermore, the entrance of blood into the lumen of a lead body can interfere with the insertion of a stylet, used for lead positioning during implantation, and with the final connection of the lead to an implantable medical device.
Methods for sealing the distal end of the lead body while still allowing a coiled conductor and drive shaft to rotate for advancing or retracting a fixation helix are known. One method is to provide a sealing membrane within the lumen of the distal lead tip. Reference is made to U.S. Pat. No. 4,311,153 issued to Smits. When the helix is advanced, the pointed tip of the fixation helix punctures the sealing membrane, which then provides a seal around the fixation helix. When used during implantation, multiple turns of the coil may be required in order to build up enough torque to overcome the friction encountered when rotating the helix through the membrane. The helix may not advance by the same amount with each turn applied to the coil. Therefore, the extension or retraction of the helix may be somewhat unpredictable. The punctured membrane may not always form a fluid-tight seal around the fixation helix. Another method for sealing the lumen of a medical lead involves positioning a sealing ring such that it encircles the drive shaft connected to the fixation helix. This type of seal may be maintained in a desired location by retainers mounted proximal and distal to the seal. Reference is made to U.S. Pat. No. 5,948,015 to Hess et al.
Infection or other changes in a patient""s medical condition sometimes necessitates the removal of a chronically implanted lead. After a lead has been implanted in a patient""s body for a period of time, fibrotic tissue growth typically encapsulates the lead, strongly adhering the lead to the surrounding tissue. Considerable traction applied to the proximal end of the lead may be necessary to pull the lead free. Reinforcement of some type extending along the lead body is beneficial in preventing breakage or partial disassembly of the lead during extraction. Several such reinforcement mechanisms are disclosed in U.S. Pat. No. 5,231,996 to Bardy et al.
In leads having an active fixation device, such as a fixation helix, the fixation device is generally housed in a relatively rigid electrode head member to provide support needed in securing the fixation device within the body tissue. The rigid electrode head member is coupled to a lead body that is more flexible for allowing easier passage through the cardiovascular structures. To improve the extractability of a lead of this type, it is desirable to transfer tensile force directly to the relatively rigid electrode head.
In the context of implantable cardiac leads, cabled or stranded conductors in place of commonly used coiled conductors provide increased tensile strength. Exemplary cabled or stranded conductors are disclosed in U.S. Pat. No. 5,760,341 issued to Laske et al., and U.S. Pat. No. 5,246,014 to Williams et al. The improved tensile strength will exist substantially between the electrode and the connector that the cabled or stranded conductor is coupled between.
Pacemaker systems, as well as other medical devices such as those mentioned above, can utilize a wide variety of lead designs. Many considerations are taken into account when optimizing the design of a lead. For example, minimizing lead size is important since a smaller device is more readily implanted within the cardiac structures or coronary vessels of a patient. Electrical insulation between multiple conductors and their associated electrodes is crucial to providing the desired therapeutic effect of electrical stimulation. Moreover, providing features that make a lead easier to implant and extract allows the clinician to complete the associated surgical procedure more safely and in less time. Finally, an optimized lead design is ideally manufactured at a low cost using techniques that are relatively simple and easy to verify. The resulting product should be easy to test so that manufacturing defects can be detected prior to the implant of the device within a patient. What is needed, therefore, is an improved lead design that takes all of the foregoing factors into account, thereby providing a medical lead that can be safely and efficiently deployed, used, and, if necessary, extracted.
The present invention is directed to a medical electrical lead system that includes a lead body having a plurality of lead body lumens, and an electrode head assembly, fixedly engaged with the lead body, having an electrode head assembly lumen with an inner wall. The electrode head assembly lumen communicating with a first lead body lumen of the plurality of lead body lumens. A first conductor extends within the first lead body lumen and the electrode head assembly lumen, an insulating member, extending through the electrode head assembly lumen and the first lead body lumen, electrically isolates the first conductor, and a drive shaft extends through the first lead body lumen and the electrode head assembly lumen. A sealing member has an outer diameter corresponding to the inner wall of the electrode head assembly lumen. The sealing member has an inner lumen that receives the drive shaft, an outer sealing member fixedly engaged with the inner wall of the electrode head assembly, and an inner sealing member engaged with the drive shaft to provide a low friction seal. A first electrode is electrically coupled to the first conductor by the drive shaft. An engaging member is positioned along the drive shaft and has a front surface. The medical electric lead system includes a flange portion that extends along the front surface of the engaging member and a retraction flange, wherein rotation of the drive shaft causes the flange portion to engage the retraction flange so that rotation of the drive shaft is absorbed by the first conductor. A second conductor extends within a second lead body lumen of the plurality of lead body lumens, and a second electrode, positioned along the electrode head assembly, has a deformation coupling the second electrode to the second conductor and transferring traction forces applied to the lead body to the electrode head assembly. A third electrode extends along the electrode head assembly and the lead body, a third conductor extends within a third lead body lumen of the plurality of lead body lumens, and an attachment member couples the third electrode and the third conductor and transfers traction forces applied to the lead body to the electrode head assembly.